The activation of lymphocytes initiates numerous events, including the synthesis of a family of lymphokines, the interferons (IFNs); these IFNs, in turn, can modulate some activities of lymphocytes. Our recent studies showed that T cells of the thymus or spleen are involved in the production of Type I IFNs stimulated by the B cell activators, lipopolysaccharide and poly l.C. The present proposal will characterize the T cell subpopulations required for these IFN responses. Additionally, the molecular and functional properties of Type I IFNs produced by T cells will be examined and compared with similarly induced B cell-derived preparations. Efforts will be undertaken to obtain purified IFN fractions from T and B cell-derived preparations and to examine the activity of such fractions on lymphocyte proliferation and antibody synthesis.